Party Leader(s)?

I have found that in most cases, regardless of which character in a group is the most likely leader, the job falls to the player who is best suited to the task. Normally by default, sometimes by design, and every now and again against the grain.

In some groups there can be several players who have the ability and desire to lead the group, while at other times you can find yourself with a group where no one is able or wishes to step up to lead.

What's the ideal situation? What's the worst situation for you as a DM or Player? What tends to be the usual situation for good or ill?